The present application also claims priority under 35 USC §119 to Korean Patent Application No. 2004-13575, filed on Feb. 27, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. A certified copy of Korean Patent Application No. 2004-13575 is contained in the parent copending patent application with Ser. No. 11/066,987.
1. Field of the Invention
The present invention relates generally to image sensors such as a CMOS (complementary metal oxide semiconductor) image sensor, and more particularly, to driving pixels of an image sensor with reduced area and high image quality.
2. Description of the Related Art
In general, an image sensor is a semiconductor device that captures images by sensing light. The image sensor includes an array of hundreds of thousands to millions of pixels that convert light of an image into electrical signals. In addition, an analog-to-digital converter converts such electrical signals that are analog signals into digital signals that are then stored in data storage units.
Semiconductor image sensors are mainly used in digital cameras and camcorders. Digital cameras for capturing still images require millions of pixels, whereas camcorders for capturing moving images require only hundreds of thousands of pixels.
Digital cameras, camcorders, and the like currently commercially available are desired to capture both still and moving images. Thus, an image sensor with millions of pixels, which is suitable for capturing a still image, is also desired to be used for capturing a moving image.
As technology advances, the number of pixels in the semiconductor image sensor is likely to increase. Conventionally, capturing a still image or a moving image using one high resolution semiconductor image sensor such as a CMOS (complementary metal oxide semiconductor) image sensor is performed in two ways.
According to one method, photocurrents from all pixels of the CMOS image sensor are measured to capture the still image. For capturing a moving image, photocurrents from selected pixels at predetermined intervals in an array of pixels are measured to capture the moving image. However, ignoring data from the non-selected pixels degrades the image quality of the moving image.
According to an alternative method also, photocurrents from all pixels of the CMOS image sensor are measured to capture the still image. However in the alternative method, photocurrents of adjacent pixels are combined to capture the moving image. Thus, because data from a substantial portion of the pixels are not ignored, the image quality is improved from the former method.
Nevertheless, a CMOS image sensor (CIS) using primary color filters cannot use the latter method. Furthermore, even when the CIS has a shared floating diffusion (FD) pixel structure, since adjacent pixels have different color filters for a Bayer color pattern, photocurrent signals from adjacent pixels cannot be combined to represent a particular color.
FIG. 1 is a circuit diagram of a conventional CMOS image sensor 100 having a plurality of pixels 101, 102, 103, 104, 105, 106, 107, and 108 and a plurality of signal converters 111, 112, 113, and 114. Each of the pixels 101, 102, 103, 104, 105, 106, 107, and 108 is arranged along rows and columns of an array of the pixels and converts a received light of a respective color into a corresponding photocurrent indicating the intensity of such received light. Each of the pixels 101, 102, 103, 104, 105, 106, 107, and 108 is comprised of a respective photodiode PD and a respective transfer MOSFET between the respective photodiode and one of the signal converters 111, 112, 113, and 114.
Each of the photodiodes PD is for receiving a respective color defined by a color filter disposed thereon. Photodiodes PD with a label R1 or R2 are for receiving red colored light, photodiodes PD with a label B1 or B2 are for receiving blue colored light, and photodiodes PD with a label Ga1, Ga2, Gb1, or Gb2 are for receiving green colored light. Each of the signal converters 111, 112, 113, and 114 converts photocurrent output from any of the pixels 101, 102, 103, 104, 105, 106, 107, and 108 coupled thereto into an output voltage Vout.
The CMOS image sensor 100 of FIG. 1 has a shared FD pixel structure in which each of the signal converters 111, 112, 113, and 114 is coupled to a corresponding pair of two adjacent pixels along a column of the array of pixels to reduce the area of the CMOS image sensor 100. For capturing a still image, the two adjacent pixels connected to a signal converter separately and sequentially output a respective photocurrent to the signal converter.
The array of pixels in the CMOS image sensor 100 have a Bayer color pattern with the pixels being for receiving alternating colors along a column or a row. Thus, the pixels 101, 102, 103, and 104 in the first column are for receiving lights of alternating colors of red, green, red, and green, respectively. Similarly, the pixels 105, 106, 107, and 108 in the second column are for receiving lights of alternating colors of green, blue, green, and blue, respectively.
Accordingly, in the CMOS image sensor 100 of FIG. 1, each signal converter 111, 112, 113, or 114 is connected to two adjacent pixels with different color filters. Thus, such a signal converter cannot combine the photocurrent signals from such adjacent pixels for simplified signal processing. That is, for capturing the moving image, the CMOS image sensor 100 would process photocurrent data from a portion of the array of pixels selected at predetermined intervals or would separately measure the photocurrents for all pixels and perform an averaging through image signal processing (ISP).
However, capturing the moving image from photocurrents of a portion of the array of pixels results in low image quality. Alternatively, capturing the moving image by separately measuring the photocurrents for all pixels and averaging through ISP requires high frequency operation and high power consumption. Nevertheless, a shared FD pixel structure is desired for reducing the area of the CMOS image sensor 100.
Thus, a mechanism for driving the pixels of an image sensor having a shared FD pixel structure with high image quality is desired.